Case Study of Vlf Tan Delta Partial Discharge

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Case Study of VLF / Tan Delta & Partial Discharge

Acceptance Testing of New 23kV Cables

Subcommittee F

Fall ICC Meeting Seattle

November 14, 2012

Timothy P Hayden PE

[email protected]


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Overview

Testing Equipment

History of VLF Testing & Procedures

Project Overview

Test Results

Failed Splice Analysis


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Testing Equipment


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Test Equipment

Second Generation

Sinusoidal Waveform

0.1 Hz maximum

60 kV DC / 43kV AC

Bluetooth communication

for Tan Delta

Partial Discharge Add On

Only 1 PD set


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History of Testing Procedures


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Introduction of VLF Testing

Major cable outage in Massachusetts in July 2003

PSC ordered review of the cause of the outage

Root cause overuse of DC testing methods on 1970s &

1980s vintage XLPE insulation

VLF testing with Tan Delta analysis introduced

Test sets purchased

New Electric Operating Procedure (EOP) developed

The new procedure required use of VLF equipment whenavailable

The procedure also limited use of DC testing equipment

Both voltage level and test time


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First Electr ic Operating Procedure 2004

First procedure used test voltage levels from 400.2

Many existing cables tested experienced a failure

New equipment (VLF) blamed for causing cable failures

Damaging the insulation

Management became reluctant to test any in service cable

Explained that the test was finding existing weak spots

Management was not convinced


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Withstand (Maintenance) Test Voltage Levels

Second procedure (2007) modified test voltage levels

No change to acceptance test levels

Withstand (or Maintenance) test broken into 2 criteria

Maximum level remained per IEEE 400.2

Minimum level based upon electric system configuration

Effectively Grounded 125% of normal L-G voltage

Delta or ineffectively grounded 100% of normal L-L voltage

Single point grounded, grounding bank, resistance grounded


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Current Testing Procedures

All new cables shall be acceptance tested

Significant length

Not a replaced section in an existing cable

When the entire circuit is new

VLF (sinusoidal) with Tan Delta analysis

0.1 Hz standard

Test voltages from IEEE 400.2

Test duration = 60 minutes


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Project Overview


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Project Overview

New 23kV Sub-transmission Cable

1000kcMil copper, 260 mils EPR, JCN (18#14 round wire),

LLDPE jacket

2 cables per phase due to loading

Spliced to existing cable after river crossing

1000kcMil copper, 260 mils XLPE, Jacketed, drain wire shield,

LLDPE jacket

New cable from supply sub across river ~ 2200

Old Cable from far side of river to next substation ~ 2500


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Plan View Not to Scale

New Manholes

New Manholes

River Crossing

Existing Manhole

& Cable


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Circuit 2339 Cable Installation

New cable was installed from the source sub

Across the river

To an existing manhole # 597

Total distance of ~ 2200

Cold shrink termination in substation

Heat Shrink End Seal in MH 597

Acceptance Tested from Substation with Tan Delta Analysis

31kV ultimate voltage, 60 minute duration

Cable sets known as 2339X and 2339Y

Therefore 6 different phases tested


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VLF / Tan Delta Test Results / Analysis


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Circuit 2339Y Tan Delta Results

New CableOnly

Suspect

moisture on

terminations

at the start

of A phase

test

Suitable for

service

TD 6.5


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Circuit 2339X Tan Delta Results

New CableOnly

Crewreportedtest set

tripped outon B phase

Crewassumedproblem

with test set B phase not

suitable forservice

TD 11

TD 46


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Circuit 2339

The test crew did not report the problem with B phase of the2339X cable to the cable crew or project management

In fact, they reported the cable as being ready for service

Accustomed to DC testing if the set doesnt trip out, cable

is OK

However, they did report the set as tripping out at least one

time

Heat shrink end seals removed and splices to old cable installed

in MH 597 Old cable length to substation ~ 2500

Total cable length now ~ 4700


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Circuit 2339 Y Old and New Cable

Old & NewCable ~ 50%

of each

5 minute test

Max 23.0kV

Acceptance

test TD was

6.5 (slide 21)

Acceptablefor service

TD 8 - 10


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Tan Delta

Tan Delta

Ratio of IRto IC

IR (leakage current) should be


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Circuit 2339 X Tan Delta Analysis

Tan Delta expected to decrease

Length of cable more than doubled

Test voltage 75% of acceptance

Tan Delta decreased by 50%

Still much higher than other phases

Original IRstill there, IC~ doubled, TD ~ halved

Test Type Length Test Voltage Phase Tan Delta

Accept 2200 31.0

A 11

B 46

C 11

Pickup 4700 23.0A 12B 24

C 12


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Circuit 2339 X Retest of B Phase Only

Retest ofcompletecable

Results still

notacceptable

TD of 17.2

SD @ 23kVwas 0.28%

Still higherthan A and Cphases

TD 17.2


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2339 X Action Plan

Project manager wanted to put cable into service

The other 2 sub-transmission cables yet to be completed

Project in danger of running behind schedule

Ultimately convinced him that this cable was not suitable for

service in current condition

If this cable were to fail while the second circuit was out of

service for this project, only a single circuit would remain

Chance of forced outages high if that were to occur

Need to remove the splice between the new cable and the

existing cable and re-test

Partial discharge test available


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Partial Discharge Test Results


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2339 X Partial Discharge Test

Splice removed between the new cable and the old cable

Cable prepped and an air termination installed

Isolated in the manhole

Partial Discharge Test performed

Using VLF at 0.1 Hz

Tested at 25.0 and 28.0 kV


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2339 X Partial Discharge Test 25.0 kV

PD test

25kV

225 feetfrom

substation

termination

Location225 feet


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2339 X Partial Discharge Test 28.0 kV

Location250 feet

PD test

25kV

250 feetfrom

substation

termination


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2339 X Partial Discharge Test

The conduit plan showed the first manhole (592) to be 150conduit feet from the riser

Due to station and manhole conditions, actual cable feet from

the manhole to the termination is closer to 200 225 feet

It become logical to suspect the splice in this manhole as beingthe problem

All cable tested in the factory

Relatively short pull length

Low chance of problem being in the cable

Splice in this manhole (592) was removed and replaced

B phase acceptance test repeated


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Circuit 2339 X B Phase after Repair

Splicereplaced

60 minute

acceptance

test

31.0kV

A phase TD

was 11.2

C phase TDwas 11.6

Acceptable

for service

TD 11.5


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Failure Analysis of Removed Splice


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Failure Analysis

Splice Body removed from cable

Left side cable Manufacturer A, Right side Manufacturer B


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Failure Analysis

Unable to determine the exact splice body alignment on thecable

Since splice body was removed prior to receiving the splice

Alignment is critical insulation interface must be fully

engaged with cable insulation However, marks remained where the splice body was positioned

Alignment was OK

There was visible tracking on the interface of side A

There were matching tracks on that side of the splice body


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Failure Analysis

Cutbacks were correct per template


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Failure Analysis

Failure believed to be due to contamination on interface Dirt from cable jacket where splice body was parked?

Improper cleaning?

Other source?


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Questions?

Documents

Case Study of Vlf Tan Delta Partial Discharge